Angle sensors, also called rotary encoders or shaft encoders convert an angular position or motion of a shaft or axle to an analog or digital code. Known angle sensors of the mechanical type may use concentric rings with openings. At each angular position, sliding contacts may detect the openings and an electronic circuit may convert the detected pattern of openings into an angular position. Optical encoders may use a similar principle. The concentric rings with openings can be replaced by a disc with transparent and opaque areas. A light source and a photo detector array make it possible to detect the pattern of opaque and transparent areas at a given angle. However, in a bearing environment with grease obscuring the disc, these types of angle sensors are difficult to use.
In bearing applications, angle sensors often comprise a ring having a plurality of magnets and one or more Hall sensors. Often, the magnetic ring is attached to a rotating part of the bearing unit. The Hall sensors, capable of detecting magnetic fields, are typically attached to the static parts of the bearing unit. The magnetic ring comprises a plurality of magnetic poles each of which provides a magnetic field. The Hall sensors detect the magnetic field and an electronic circuit coupled to the Hall sensors converts the detected magnetic field to an angular position within each of the plurality of magnetic poles. This arrangement also works in the greasy bearing environment.
Such angular sensors are, for example, known from a US patent application US2009315544. The angular encoders according to this patent application comprise a rotation detection device which includes a plurality of magnetic encoders of a ring shape arranged coaxially and having different numbers of magnetic poles, a plurality of magnetic sensors each operable to detect the magnetic field of the corresponding magnetic encoder and having a function of detecting positional information within a single magnetic pole of the corresponding magnetic encoder, a phase difference detector for determining the phase difference of magnetic field signals detected respectively by the magnetic sensors, and an angle calculator for calculating an absolute rotation angle of the magnetic encoders based on the detected phase difference.